The present development is a catalyzed diesel particulate matter exhaust filter with improved thermal stability and improved diesel particulate matter oxidation activity. The catalyzed filter comprises a porous filter substrate for filtering the diesel particulate matter washcoated with high surface area support. Exemplary supports comprise alumina, titania, silica, zirconia or a combination thereof promoted with ceria, lanthanum oxide, tungsten oxide, molybdem oxide, tin oxide or a combination thereof. The catalytic materials of the filter can include an alkaline earth metal vanadate and a precious metal.
Internal combustion engines function by burning fuels (hydrocarbons) at high temperatures. In theory, the products of the combustion process are CO2 and water. But, it is not uncommon that the combustion process is incomplete resulting in the formation of undesirable byproducts are formed such as carbon monoxide, hydrocarbons and soot. Other reactions occurring in internal combustion engines include the oxidation of nitrogen molecules to produce nitrogen oxides and the oxidation of sulfur to form SO2 and small percentage of SO3. Further, when the temperature decreases, the SO3 can react with H2O to form sulfuric acid. Other inorganic materials are formed as ash. The products of these reactions result in undesirable gaseous, liquid and solid emissions from internal combustion engine: gaseous emissions—carbon monoxide, hydrocarbons, nitrogen oxides, sulfur dioxide; liquid phase emissions—unburned fuel, lubricants, sulfuric acid; and, solid phase emissions—carbon (soot). The combination of liquid phase hydrocarbons, solid phase soot and sulfuric acid results in the formation of small size droplets often called total particulate matter.
The most common types of internal combustion engines are diesel engines and Otto engines. Compared with Otto engines, diesel engines emit more particulate matter and pose a greater threat to air quality and to the health of human beings. To reduce these risks, tremendous efforts have been made for the control of diesel particulate emissions. One well known approach is to use filters to trap exhaust particulate matter. These filters are generally made of porous, solid materials having a plurality of pores extending therethrough and small cross-sectional sides, such that the filter is permeable to the exhaust gas which flows through the filter and yet capable of restraining most of all of the particulate materials. As the mass of collected particulate material increases in the filter, the flow of the exhaust gas through the filter is gradually impeded, resulting in an increased backpressure within the filter and reduced engine efficiency.
Conventionally, when the backpressure reaches a certain level, the filter is either discarded, if it is a replaceable filter, or removed and regenerated by burning the collected particulate materials off at temperatures of from about 600° C. to about 650° C. so the filter can be reused. Regeneration of filters in situ can sometimes be accomplished by periodically enriching the air fuel mixture. The enrichment produces a higher exhaust gas temperature. The high exhaust temperature burns off the particulate materials contained within the filter.
Thermal regeneration of diesel particulate filter at temperatures above 600° C. is not generally desirable because it can lead to uncontrolled lightoff of soot, temperature overshoot and damage of the filter substrates. In addition, thermal regeneration consumes large amounts of energy. Rather, regeneration of diesel particulate filters at lower temperature is preferred. Such regeneration can be accomplished with the assistance of catalysts. For example, U.S. Pat. Nos. 5,100,632 and 4,477,417 each teach a catalyzed filter that will regenerate at temperatures lower than 600° C.
Several patents teach compositions for diesel exhaust particulate filters. Many of the compositions use a combination of particular vanadium compounds with a platinum compound. For example, U.S. Pat. No. 4,510,265 discloses a coated diesel particulate filter formed by coating a solution comprising a platinum group metal and a silver vanadate onto a ceramic monolithic support material. Another diesel exhaust particulate filter is disclosed in U.S. Pat. No. 4,588,707 in which a catalytically active substance formed from lithium oxide, copper chloride, a vanadium oxide/alkaline metal oxide combination or precious metal is coated onto a filter substrate.
U.S. Pat. No. 5,514,354 teaches an open cell monolithic catalyst for the purification of diesel exhaust gases. The monolith is coated with oxides containing vanadium and platinum group metals as active components
U.S. Pat. No. 6,013,599 discloses a diesel particulate filter which can be regenerated in situ, which is formed from a porous refractory support material onto which washcoating is secured. The washcoating in one embodiment is formed by mixing an acidic iron-containing compound and a copper-containing compound, adding an aqueous alkaline metal salt solution and an acidic vanadium-containing compound and finally adding to the mixture an alkaline earth metal compound slurry.
While these patents disclose a number of different compositions of material for use as filters for diesel particulate matter, there are still significant problems associated with the increased pressure drop experienced during use of these filters. Furthermore, the thermal stability of the catalysts decrease with increasing temperature if the catalysts are directly coated on to low surface area substrates. The catalysts sinter and deactivate at high temperature during regeneration. In addition, some of the diesel combustion catalysts do not have good sulfur poison resistance and can be deactivated at high temperature.